Butterflies move north due to hot summers

Once rare UK butterflies have been moving further north due to a pattern of hot summers, say researchers.

The brown argus, one of Britain's smallest butterflies, was limited to sun-loving shrubs on chalk grassland.

But since the 1990s the species has been expanding its range, using different plants to host their eggs.

Scientists from the University of York attribute this to rising summer temperatures.

Analysing records made by volunteers with the charity Butterfly Conservation, Rachel Pateman and colleagues at the University of York and the NERC Centre for Ecology and Hydrology identified that populations of the butterfly have spread 79km (49 miles) further north in 20 years.

"The butterfly is now present throughout much of the south and east of England as far north as South and East Yorkshire," explained Ms Pateman whose study was published in the journal Science.

"This is unusual for species which are considered fairly scarce and specialised and so we were interested to discover what might have caused this rapid range expansion."

The amateur records held another clue for researchers as the butterflies seemed to have broadened their taste in host plants. In the past the insects only fed upon rockrose, now their tastes include Geraniums such as dove's-foot cranesbill.

"Warm summers have benefited the brown argus butterfly by allowing it to become less picky," Ms Pateman told BBC Nature.

"The Geranium species used are very widespread in the landscape and so this has enabled the rapid expansion of the butterfly."

According to the scientists, rising summer temperatures are responsible for the warmth-loving butterflies' success.

Their investigation of climate data for the UK identified the link: summer temperatures from 1990 to 2009 were on average 0.78% warmer than previous summers, dating back to 1800.

"We think the brown argus was previously restricted to using rockrose because it grows in places with particularly warm microclimates and therefore was suitable when conditions were particularly cool," Ms Pateman said, explaining that the yellow-flowered shrub is typically found in "short turf on southerly-facing slopes."

"Geranium species, however, tend to grow in cooler places and so were unsuitable for brown argus when the climate was cooler," she said.

"As temperatures have increased we think the locations where Geranium species grow have now become warm enough for the brown argus."

Researchers focussed on the counties of Suffolk and Bedfordshire in their study, where recording the orange spotted wings of the brown butterflies is particularly popular.

"There has been an increase in recording over the years. However, when methods have been used to control for this, an increase in the distribution of brown argus is still found," said Ms Pateman.

"The records submitted by volunteers are an extremely valuable resource, without which the detection of long-term patterns like this would not be possible."